A sheet-shaped separator is interposed between the positive electrode and the negative electrode of a lithium ion secondary battery. The separator serves to provide an electrical insulation between the positive electrode and the negative electrode, and to retain the electrolyte. A microporous film is used for the separator of the lithium ion secondary battery. The microporous film can be mainly obtained by molding a resin or a resin composition into a sheet, and further drawing it. Although there is no particular limitation with respect to the resin serving as the source material of the microporous film, a polyolefin resin (e.g., polyethylene or polypropylene) is widely used.
A microporous film as described above tends to shrink at high temperatures. Therefore, when an internal short circuit has occurred to cause heat generation, the microporous film shrinks, resulting in the expansion of the short circuit portion. The expansion of the short circuit portion leads to a further generation of reaction heat resulting from short circuit. Accordingly, there is the possibility that the temperature inside the battery may increase abnormally.
Therefore, from the viewpoint of suppressing the expansion of the short circuit portion and to improve the safety of the battery, it has been proposed to form a porous insulating layer including an inorganic filler and a resin binder on the electrode active material layer (Japanese Patent No. 3371301). The porous insulating layer is filled with an inorganic filler. The particles of the filler are bound to one another with a relatively small amount of a resin binder. Accordingly, the porous insulating layer tends not to shrink even at high temperatures. The porous insulating layer has a function for suppressing the expansion of the short circuit portion at the time of occurrence of an internal short circuit.